There's been a lot of discussion on AGA lately about the Marshall 2204 "Cascaded" preamp circuit and whether or not it sucks and how much. A lot of people really do like the high gain preamp. But what sucks about the cascaded Marshall circuit is that there's no way to switch the high and low gain besides physically moving a cable from one input jack to the other. That circuit was a long time ago, though, these days everyone likes channel switching amps which use relays or other switching logic to push the signal through different tube stages and recombine them at the power amp. Which is complicated and no fun to homebrew.
Anyway I came up with this circuit, which allows easy footswitching with no relays or anything between a 'clean' input stage and a 'high gain' input stage, with appropriate frequency response voicing. It's built and working, as part of a homemade amplifier, and I really do like the sound so far.
I don't know if it's original or not, in the context of a guitar amp, but I think it's pretty neat.
This circuit pretty much looks like a Marshall "cascaded" preamp here. But with the footswitch open, the 47K resistor and 0.44 uF capacitor form a feedback loop between the plate of the second triode (half of a 12AX7) and the cathode of the first triode. The 0.44 uF capacitor really doesn't do anything here but just pass the feedback signal through - the DC is already blocked by the 0.022 uF coupling capacitor between the second triode's plate and the output load, and any high-pass filtering is going to be dominated by the 0.022 uF cap as well. The result of the feedback loop is a stage with a moderate amount of gain, pretty flat frequency response, and low distortion unless driven to the limits where it'll hardclip, which a typical guitar pickup won't be able to do. So it's a typical 'clean' input stage.
Now close the footswitch, this grounds the middle of the feedback loop and breaks it. The 0.44 uF capacitor now becomes a cathode bypass capacitor for the first triode, bringing in a gain boost above about
300 Hz. The 47 K resistor becomes just a dummy load hanging off the output. Without feedback, the stage distorts much more readily, gain is much higher, and the frequency response is not as even. Lows are rolled off by the interstage coupling RC network and the treble rolls off due to Miller effect capacitance. It ends up being a typical 'high gain' input stage with appropriate frequency response and distortion. Crunch!Small-signal SPICE simulation of the frequency response and gain of each stage is here:
What makes this really amenable to footswitching is that although the gain at the switching point is high, the impedance is very low, so noise pickup isn't much of an issue. The feedback loop keeps the impedance low to such noise signals, it's sort of a virtual ground. So I just think this circuit is really neat.
There's a lot of talk on AGA about the differences in voicing a clean and distorted channel. It's true that you can't totally tweak everything here, but you can probably do quite a bit to change frequency response and gain. The feedback resistor could probably even include a variable pot.